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CON 124 - Session 6 - Concrete Durability
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CON 124 - Session 6 - Concrete Durability

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O. Tavares

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    CON 124 - Session 6 - Concrete Durability CON 124 - Session 6 - Concrete Durability Presentation Transcript

    • CON 124 Basic Concrete Mix Design Proportioning Session 6 Concrete Durability
    • Session 6 Topics       Alkali-Silica Reactivity (ASR) Water/Cementitious Ratio Shrinkage Mechanisms Testing procedures Mitigation measures
    • Alkali-Silica Reactivity
    • Alkali-Silica Reactivity (ASR)… ACI 116   The reaction between the alkalies in portland cement and certain siliceous rocks or minerals present in some aggregates The products of the reaction may cause abnormal expansion and cracking of concrete...
    • Alkali-Silica Reaction (ASR) • Contributing factors: – Reactive forms of silica in the aggregate – High-alkali (pH) pore solution – Sufficient moisture Crack Reaction Product
    • Mechanism Reactive silica + alkalis Alkali-silica gel OH- OH- Reactive Silica OH- OH-
    • Mechanism Silica gel + Water Expansion H2O H2O Reactive Silica H2O H2O
    • Mechanism Expansion  Cracking H2O H2O Reactive Silica H2O H2O
    • Cracking and Appearance of ASR Gel
    • Alkali-Silica Reaction (ASR) Visual Symptoms • Network of cracks • Closed or spalled joints • Relative displacements
    • Alkali-Silica Reaction (ASR) Visual Symptoms  Fragments breaking out of the surface (popouts) Mechanism  Alkali hydroxide + reactive silica gel x reaction product (alkali-silica gel)  Gel reaction product + moisture x expansion
    • Alkali-Silica Reaction (ASR) Visual Symptoms  Fragments breaking out of the surface (popouts) Mechanism  Alkali hydroxide + reactive silica gel x reaction product (alkali-silica gel)  Gel reaction product + moisture x expansion
    • Mitigation of ASR    Avoid reactive aggregates Limit concrete alkali content, 5 lbs/cyd (low alkali cement) Supplementary cementitious materials    Fly ash, slag, calcined clay (metakaolin) Blended cement Test for effectiveness of mitigation measures
    • Aggregates   The field-performance record of a particular aggregate is best means for judging its reactivity If aggregates are shown by service records or laboratory examination to be potentially reactive, they should not be used when the concrete is to be exposed to seawater or other environments
    • ASR – The Key Components Alkalis in Portland Cement    Cement: Sodium and Potassium Expressed as (Na2O + 0.658K2O) ASTM C 150 - Table 2, Optional Chemical Requirements   Low-Alkali Cement - Max 0.60% Low-alkali cement to be used with reactive aggregates (ASTM C 33)
    • ASR – The Key Components & Other Sources of Alkalis in Concrete  Supplementary Cementitious Materials      Fly Ash Slag Cement Silica Fume High-Reactive Metakaolin Sodium-Bearing Admixtures
    • Alkali-Silica Reactivity The correlation at 14 weeks is not as great as at later ages.
    • Cement – Aggregate Test ASTM C227 0.50 Na2Oeq=0.92% 6-months Expansion, % 0.40 Na2Oeq=0.57% 0.30 0.20 0.10 0.00 A B Aggregate ASR Test Method alkali content and aggregate comparisons C227
    • Tests for ASR Potential        ASTM C227—Mortar Bar Method ASTM C441—Mortar Bar and SCMs ASTM C289—Mortar Bar Method ASTM C1260—Rapid Mortar Bar Test ASTM C1293—Concrete Prism Test ASTM C1567—Rapid evaluation of SCMs …Others
    • Exposure Methods   Warm and humid environment— “over water at 38 °C” Accelerated conditions—immersed in NaOH solution at 80 °C
    • ASTM C227, C441 & C1293 “Over Water @ 38 °C” Sealed container in 38 C room Water Wicking material on inner surface
    • ASTM C1260 & C1567 “Immerse in NaOH at 80 °C” Polypropylene box with cover Water Bath 80 2 C 1N NaOH Solution
    • ASTM Test Methods Comparisons ASTM Test Use Specimen Type Test Condition Common Duration C 227 Cem-Agg Combination Mortar Over H2O @ 38 °C 90, 180 d C 441 Effectiveness of SCMs Mortar (Pyrex + HA Cement) Over H2O @ 38 °C 14, 56 d C 1260 and [C 1567] Aggregate [Effectivenesof SCMs] Mortar Immerse in NaOH @ 80 °C 16 d C 1293 Aggregate [Effectiveness SCMs] Concrete (Added NaOH) Over H2O @ 38 °C 1 or 2 yr
    • ASTM C295
    • ASTM C295
    • Effect of Fly Ash on ASR (Class F) (ASTM C1293) 0.10 % Expansion 0.08 Control 15 % FA 25 % FA 0.06 1 yr limit 0.04 0.02 0.00 0 200 400 600 Time, days 800 1000 1200
    • Effect of Fly Ash – Aggregate R and Cement NA 14-day Expansion, % 0.20 Fly Ash H (CaO=18.6%) 0.15 Fly Ash L (CaO=2.4%) 0.10 0.05 0.00 0 20 Fly Ash, % 25 Effect of Fly Ash on aggregate, Fly Ash H is a Class C, whereas Fly Ash L is a Class F
    • Effect of Slag on ASR (ASTM C1293) 0.10 % Expansion 0.08 0.06 1 yr limit 0.04 Control 25 % Slag 40 % Slag 50 % Slag 0.02 0.00 0 200 400 600 800 Time, days 1000 1200
    • Effect of SCMs ASR Influence of different amounts of Class F fly ash, slag, and silica fume by mass of cementing material on mortar bar expansion (ASTM 1260) after 14 days when using reactive aggregate.
    • Differences in Specifications for Effectiveness of SCMs for ASR      ASTM C595 (blended cement) ASTM C1157 (performance hydraulic cement) ASTM C989 (slag cement) ASTM C618 (fly ash and natural pozzolans) ASTM C1240 (silica fume)
    • Test Methods Comparisons ASTM Specification Test Method Specification Limit Test Control C595 and C1157 C227 (Pyrex) 0.020 % @ 14 d N/A C989 C441 0.020% @14 d or 75 % reduction High alkali cement C618 C441 (as modified in C311) Max. 100% of Control @14 d Low alkali cement < 0.60% C441 At least 80% reduction @ 14 d High alkali cement C1240
    • The Effect of Water-To-Cementitious Ratio Law “For given materials the strength of the concrete (so long as we have a plastic mix) depends solely on the relative quantity of water as compared with cement and/or cementitious regardless of mix or size and grading of aggregate.” --Duff A. Abrams May 1918
    • Water Cementitious Ratio    Strength increases as the w/cm ratio decreases Concretes with the same w/cm ratio but different ingredients are expected to have different strengths A lower w/cm ratio reduces set time
    • Relationship of Water to cement or cementitious ratio, as W/cm ratio decreases strengths increase.
    • The strength relationships holds true at various ages, 7-day and 28-day respectively
    • As the w/cm ratio increase so does the volume does the volume of concrete
    • Regardless of w/cm ratio, the weight of concrete is constant
    • Effect of w/cm The type of cement does not impact concrete durability as much as water to cementitious ratio, irregardless of the type of concrete specified 5 4 Visual Rating 3 w/cm = 0.38 w/cm = 0.47 w/cm = 0.68 2 1 0 2 4 6 8 10 12 14 16 Age, years Types I, II, V, blended cements, pozzolans, slag
    • Effect of Cement Type 5 Type V (4 % C3A) Type II (8 % C3A) Type I (13 % C3A) 4 Visual Rating 3 2 1 2 w/c = 0.38 4 6 8 10 Age, years 12 14 16
    • Effect of w/cm Type V Cement w/c = 0.65 Visual Rating = 5 @ 12 years Source: PCA Type V Cement w/c = 0.39 Visual Rating = 2 @ 16 years Effect of w/c comparing Type V cement at various w/cm ratios, higher w/cm ratio, concrete less durable
    • Durable Concrete    A low w/cm will produce less permeable concrete and provide greater protection against aggressive environmental conditions A w/cm of 0.40 and adequate cover over the steel performs significantly better than concretes made with w/cm of 0.50 and 0.60 Frost-resistant normal weight concrete should have a w/cm not exceeding 0.50
    • Schematic of Total Shrinkage
    • Shrinkage  Volume Reduction due to loss of moisture from a concrete matrix as it hardens and dries.      Plastic Shrinkage Thermal Contraction Drying Shrinkage Autogenous Shrinkage Settlement Shrinkage
    • Drying Shrinkage Mechanism   Loss of moisture Meniscus forms at air-water interface due to surface tension
    • Drying Shrinkage Mechanism   Surface tension forces exert inward pulling force on the walls of the pores Most significant in pore sizes ranging from 2.5-50 nm Capillary Tension
    • Reducing Total Shrinkage Potential     Keeping the water (or paste) content low The higher the cement content of a mixture the higher shrinkage Increase Coarse Aggregate Content Avoiding aggregates that contain excessive amounts of clay in their fines
    • Additional Concerns on Shrinkage    High concrete temperatures demand increased water demand Reduction in the effectiveness of the air-void system due to higher temperatures Winter effects the primary danger is that low temperatures may hydrate slower
    • Rate of Hydration     Early strength of a concrete mixture will be higher with an elevated temperature Chemical reactions are faster at higher temperatures With increasing temperature, the potential for an imbalance in the cementitious paste system will be exacerbated Cement fineness affects the rate of heat generation
    • Rate of Heat Loss     Influenced by the thickness of the concrete sections Thinner concrete sections will not get as hot as thicker sections Thermal expansion and contraction of concrete depends on concrete mix design Coefficient of thermal expansion (CTE) changes in length (or volume) for a given change in temperature dependent on aggregate CTE
    • Please return to Blackboard and watch the following videos:  Video 1: Durability
    • Questions? Email cemtek@netzero.net